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Erik Malund
03/26/06 10:55
  03/26/06 10:58

Read: 433 times
Mt Airy, Nc

#113144 - "expensive" is relative
Responding to: Bruce Grenade's previous message
Why do I never see engineers use BJT's for switching off supplies? I've seen many times where an engineer specs an expensive MOSFET in situations where the "On resistance" doesn't matter and the circuit they are switching off is well below general purpose current levels.

sometimes the voltage drop is important, sometimes the heat, sometimes the speed, sometimes ...

Do not jump the gun and say "unnecessary" for the design of others.

If you used a mosfet, you could reduce/omit the capacitor which gives you some very undesirable states while discharging after an "off" command, maybe you can live with them, maybe those you critisize can not. Also, there is quite a stress on the transistor when an "on" command suddenly tries to charge the cap, you may need current limiting to make your PNP stay alive"


PS I use no such designs, not defending myself.

List of 7 messages in thread
Switching an IC off using BJT?      Bruce Grenade      03/25/06 21:36      
   Try using a PNP        Russell Bull      03/26/06 00:49      
   Good Good      Bruce Grenade      03/26/06 09:16      
      "expensive" is relative      Erik Malund      03/26/06 10:55      
         Of course      Bruce Grenade      03/26/06 11:47      
   Using a MOSFET makes sense        Kai Klaas      03/26/06 16:28      
      Good Info      Bruce Grenade      03/26/06 17:03      

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